Pion pair production in photon-photon collisions at DCI

This paper presents experimental results on π⁺π⁻ production threshold from the collision of quasi-real photons. The data, obtained at the e⁺e⁻ collider DCI, are a combination of the results from the DM1 and DM2 experiments. Using the e⁺e⁻ and π⁺π⁻ production for normalization and cross-checks, we observe a pion pair yield at low invariant mass (W < 500 MeV/c²) which is approximately twice the one expected from Born terms.     

Pion pair production in photon-photon collisions at spear

We report a measurement of the cross section for the process γγ→π⁺π^? at invariant masses 500 MeV/c² < m_ππ<2000 MeV/c². A value for the radiative width of the f(1270) tensor meson Γ_f→γγ = 3.6 ± 0.3 ± 0.5 KeV (helicity λ = 2) has been obtained from a fit to the observed ππ mass spectrum.     

Multipion production in photon-photon collisions

The magnitudes of the matrix elements for γγ→2π⁺2π^?, γγ→π⁺π^?2π⁰, γγ→2π⁺2π^?π⁰, γγ→2π⁺2π^?2π⁰, γγ→3π⁺3π^?3π⁺ and γγ→3π⁺3π^?π⁰are deduced from experimental data as a function of the invariant massW of the system (from 1.0 to 3.0 GeV). It is shown that the data are consistent with a universal energy dependence of the reduced channel amplitudes. A statistical model is shown to describe the peaks in the energy dependence of multipion production in γγ collisions quite well without introducing explicit intermediate resonances.     

Two-pion production in photon-photon collisions

We calculate within chiral perturbation theory the cross section for γγ→π⁰π⁰ and γγ→π⁺π⁻ to next-to-leading order. The first process only depends on the one-loop contributions and forms a test of chiral perturbation theory. We also calculate the pion electromagnetic form factor and compare it to recent data.     

Pion production in nuclear collisions

The production of pions in nuclear collisions is analyzed in terms of a microscopic reaction model, where the free energy of the entrance channel is transferred to the final channel through the coupling of the relative motion to the internal excitation of N^*-resonances. Such a model allows quite naturally for a consistent analysis of the exclusive and the inclusive production of pions. It turns out that the pionic fusion cross section is determined predominantly by the spectroscopic parentage between the initial target projectile combination and the final nucleus, whereas the inclusive part is dominated by the energy dependence of the phase space factor, i.e. by the number of the degrees of freedom which are relevant in a given kinematical situation. This model is applied to the analysis of available experimental data from threshold to the (1232)-region. A consistent analysis of both the inclusive and the exclusive part of the pion spectrum is presented.Dedicated to Prof. Dr. P. Kienle on the occasion of his 60th birthday     

Pion production in two-body pd collisions

The reactions d(p, π⁺)t and d(p, π^o)³ He are analyzed in terms of the pp → π⁺d sub-process. The exclusion principle induces an additional exchange contribution involving the πd scattering amplitude. Acceptable deuteron and tritron (³H) wave functions which reproduce the charge form factors yield results consistent with experimental angular distributions at 470, 590, 754 and 1192 proton lab. energies. The forward peak is not sensitive to details of the wave functions. Both the direct and exchange form factors develop a zero which is imposed by the dip in the ³He charge form factor. The associated interference pattern governs the backward cross section.     

Pion pair production with higher order radiative corrections in low energy $e^+e^-$ collisions

The complete QED initial state (IS), final state (FS) and initial–final state (IFS) interference corrections to the process are presented. Analytic formulae are given for the virtual and for the real photon corrections. The total cross section (), the pion angular distribution () and the invariant mass distribution () are investigated in the regime of experimentally realistic kinematical cuts. It is shown that in addition to the full corrections also the and leading-log photonic corrections as well as the contributions from IS -pair production have to be taken into account if at least per cent accuracy is required. For the data analysis we focus on an inclusive treatment of all photons. The theoretical error concerning our treatment of radiative corrections is then estimated to be 2 per mill for both the measurement of the total cross section and the invariant mass distribution. In addition we discuss the model uncertainty due to the pion substructure. Altogether the precision of the theoretical prediction matches the requirements of low energy experiments like the ones going on at DANE or VEPP-2M. Received: 6 September 2001 / Revised version: 24 January 2002 / Published online: 5 April 2002     

High-twist correction for inclusive hadron production in photon-photon collisions

The contribution of the QCD high-twist subprocess γq→πq′ to the inclusive cross section for e⁺ e^? → e⁺ e^? + π^± + X is calculated both in the PETRA/PEP and the LEP energy range, in the equivalent-photon approximation. Superposed on the Born cross section and its O(α_s) correction, it gives agreement with the data at relatively low transverse momenta. It is suggested that this high-twist contribution may be further isolated experimentally by triggering simultaneously on both a fast hadron and the opposite-side jet.